James T. Shipman
Jerry D. Wilson
Charles A. Higgins, Jr.
Omar Torres
Chapter 6
Waves and Sound
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Waves
• We know that when matter is disturbed, energy
emanates from the disturbance. This propagation of
energy from the disturbance is known as a wave.
• Waves transfer energy form place to place but do not
transfer matter (in general)
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Section 6.1
Longitudinal & Transverse Waves
There are two types of waves, classified based
on their particle motion and wave direction:
Longitudinal Wave: (sound):
The particle motion and the wave velocity have the same direction
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Section 6.2
Longitudinal & Transverse Waves
Transverse Wave (light):
The particle motion is perpendicular to the direction of wave velocity
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Wave Description
• Wavelength (l) – the distance of one complete wave
• Amplitude – the maximum displacement of any part of the
wave from its equilibrium position
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Section 6.2
Period and Frequency
Period (T ) : the time it takes for a wave to travel a one wavelength
Frequency ( f ) : the number of oscillations during 1s, its unit is hertz (Hz)
•
Frequency and Period are inversely proportional, f 
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1
T
Period and Frequency
Find the period and Frequency for each wave?
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Section 6.2
Wave Speed (v)
• Since speed is distance/time:
v = l/T or v = lf,
v = wave speed (m/s)
l = wavelength (m)
T = period of wave (s)
f = frequency (Hz)
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Section 6.2
Calculating Frequency –
Confidence Exercise
A sound wave has a speed of 344 m/s and a wavelength of
0.500 m. What is the frequency of the wave?
Solution:
Rearrange formula (v = lf ) to solve for f = v/l
f = v/l = (344 m/s)/(0.500 m/cycle)
f = 688 cycles/s
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Section 6.2
Electromagnetic Waves
• Consist of vibrating electric and magnetic fields that
oscillate perpendicular to each other and the direction of
wave propagation. The speed of electromagnetic waves is
c = 3.00×108 m/s
A
-A
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Section 6.3
Electromagnetic (EM) Spectrum
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Computing Microwave Frequency
If the wavelength of a microwave beam is 11.5 cm,
then what is the frequency of the radiation?
3.0 108 m / s
f  
l 11.5 cm  1m
100cm
c
 2.6 1011 s 1
Computing Radio Wave Wavelength
What is the wavelength of the radio waves produced
by a station with an assigned frequency of 600 kHz?
Solution:
f = 600 kHz = 600×103 Hz = 6.00×105 Hz
Rearrange equation (c = lf ) and solve for l
l = c/f = (3.00×108 m/s)/(6.00×105 Hz)
l = 0.500×103 m = 500 m
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Section 6.3
Calculating visible wavelengths
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Sound Waves
• Sound:
− the propagation of longitudinal waves through matter
(solid, liquid, or gas)
− The vibration of a tuning fork produces a series of
compressions (high pressure regions) and rarefactions
(low pressure regions)
Section 6.4
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Sound Spectrum
• Similar to the electromagnetic radiation,
sound waves also have a spectrum
• The sound spectrum can be divided into
three frequency regions:
– Infrasonic, f < 20 Hz
– Audible, 20 Hz < f < 20 kHz
– Ultrasonic, f > 20 kHz
• The audible region for humans is about
20 Hz to 20 kHz
• Sounds can be heard due to the vibration of
our eardrums caused by the sound waves
propagating disturbance
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Section 6.4
Sound Intensity
• Loudness or Sound intensity (I) is the rate of energy
transfer through a given area, has a unit of J/s/m2 or W/m2
• Sound Intensity decreases inversely to the square of the
distance from source (I  1/r2)
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Section 6.4
Decibel Scale
• Sound Intensity is measured on the decibel scale
• A decibel is 1/10 of a bel (in honor of Alexander Graham Bell)
• The decibel scale is not linear with respect to intensity
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Section 6.4
The Doppler Effect Illustrated
The Doppler effect: the apparent change in frequency resulting
from the relative motion of the source and the observer
Lower Frequency
Higher frequency
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Section 6.5
The Doppler Effect
• As a moving sound source approaches an observer, the
waves in front are bunched up and the waves behind are
spread out due to the movement of the sound source
• The observer hears a higher pitch (shorter l) as the sound
source approaches and then hears a lower pitch (longer l) as
the source departs
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Section 6.5
Sonic Boom
• As the jet approaches the speed of sound, compressed sound waves
and air build up and act as a barrier in front of the plane
• As a plane exceeds the speed of sound it forms a high-pressure shock
wave, heard as a ’sonic boom’
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Section 6.5
Standing Waves
• Standing wave – a “stationary” waveform arising from the
interference of waves traveling in opposite directions
– When these two waves meet they constructively “interfere” with
each other, forming a combined and standing waveform
λ
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Section 6.6
Resonance
• When one tuning fork is struck, the other tuning fork of the
same frequency will also vibrate in resonance
• The periodic “driving force” here are the sound waves
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Section 6.6
Homework (Exercises)
1
4
6
11
14
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